The invention relates to an arrangement for the recording of rastered half tone pictures in accordance with the above referred to U.S. Pat. No. 3,725,574, in which the exit openings for the emergence of the respective light beams from the recording apparatus are coordinated in a line.
The reference patent illustrates an arrangement for the so-called "Electronic Rastering of Half Tone Picture", by means of which each covering point, i.e. rastered point in the picture reproduction, is composed of a number of recording lines running in a direction parallel to the recording direction. The recording lines are simultaneously exposed upon a recording medium, such as a film, by means of a plurality of beams derived from a laser light source.
The form and size of covering points thus are dependent upon the length of the individual recording lines making up the same, with the lengths of the recording lines being controlled by suitable means such as light modulators and the corresponding recording data whereby the length of the partial beams can be suitably controlled.
The recording data for the respective covering points is stored in a digital store of a raster point computer for each possible tonal value and for various raster angles. During the recording of a covering point, the transformation of the tonal value information of the picture point involved of the pattern is effected in the raster point computer into recording data for the control of the light modulators.
The modulated individual or partial beams may exit from openings in the recording device over photoconductors, for example, to provide an arrangement such as illustrated in FIG. 6 of the reference patent with the exit openings being arranged in a line extending transversely to the recording direction, and reproduced on the film by means of an objective lense.
When the individual or partial beams are turned on, adjoining light points are formed on the film which expose parallel recording lines of a covering point, as a result of the movement of the film in the recording direction. As the light points are arranged from one another at distance for example, as a result of a coating or covering of the individual photoconductors, which can be even irregular as a result of production tolerances, while the center of a recording line is fully exposed by the spaced light points, the border zones are only exposed by stray light, as a result of which an inhomogenous density may be formed in the exposure profile across a covering point, which often is undesirable.
A subsequent tonal value correction of an already recorded rastered half tone picture, more particularly a so-called point or subsequent etching by means of a reducing agent, is carried out in the reproduction technique. It involves a chemical process in which the covering points in the tonal picture or in partial areas are reduced as to their surface area by removing some of the deposited silver. However, in this subsequent treatment, the density in the center of a covering point is simultaneously decreased. With a continuous effect of the reducing agent, the covering points with an inhomogenous exposure profile finally disintegrate into the individual recording lines and the half tone picture loses its copying properties.
Attempts are made in actual practice to avoid such a stripped formation of the covering points by the utilization of overexposures and by utilizing films with a steep graduation (lith-films). However, this achieves only partial success as the effectiveness of such measures are primarily dependent to a great extent upon the exposure and development of the films. Further, it involves processes which normally require much experience and which is considered particularly disadvantageous. In addition, such process contains the further disadvantage that the specified measures for decreasing the stripping structure can be utilized only in certain specific film materials.